1. Field of the Invention
The present invention relates to a method of manufacturing an optical fiber cable covered with a metal pipe, and an apparatus for manufacturing this optical fiber cable.
2. Description of the Related Art
When a tension is applied to an optical fiber having a diameter of 250 .mu.m, although a breaking strength is as rather large as about 6 kg, the elongation is 3 to 6%. This elongation is considerably small as compared to that of a conventional copper or aluminum cable. For this reason, a tensile strength member must be provided to the optical fiber to maintain a high optical fiber cable strength. When an optical fiber is dipped in water, its strength is sometimes degraded. Therefore, when laying an optical fiber cable underseas or underwater, an optical fiber cable having a sheath structure obtained by covering an optical fiber with a thin metal pipe must be employed to maintain a high laying tension and water resistance.
Conventionally, when such an optical fiber having a small diameter is to be covered with a metal pipe, the optical fiber is inserted in a metal pipe having a gap in the longitudinal direction, and this gap is welded by soldering. According to this method, however, heat generated during welding of the metal pipe is applied to the optical fiber through the gap for a comparatively long period of time, leading to thermal damage to the optical fiber.
Jpn. Pat. Appln. KOKAI Publication No. 64-35514 discloses an apparatus and method of continuously manufacturing an optical fiber covered with a metal pipe by welding the abutting portion of the metal pipe with a focused laser beam, so that thermal damage in the optical fiber is prevented. In this apparatus for manufacturing an optical fiber cable covered with a metal pipe, a flat metal strip which is continuously supplied is formed into a metal pipe having a longitudinal gap at its top portion. An introducing tube is inserted in the metal pipe through the gap in the metal pipe, and an optical fiber is inserted in the metal pipe through the introducing tube. After the gap of the metal pipe in which the optical fiber is introduced is closed, the metal pipe is supplied to a laser welding unit. The laser welding unit irradiates a laser beam having a focal point at a position outwardly remote from the surface of the abutting portion while positioning the abutting portion at the top portion of the supplied metal pipe with a guide roller, thereby welding the abutting portion. In this manner, welding of the abutting portion is realized by shifting the focal point of the laser beam from the abutting portion without protecting the optical fiber with a heat-shielding member. Subsequently, the outer diameter of the metal pipe incorporating the optical fiber is reduced to a predetermined size, and the metal pipe is wound on a capstan and continuously withdrawn from the capstan.
In withdrawal of the metal pipe, an inert gas is supplied to the introducing tube. The optical fiber is transported into the metal pipe with thee viscosity resistance of the inert gas. While the metal pipe is engaged with the capstan, the optical fiber is positioned on an outer side of the inner portion of the metal pipe by blowing the inert gas. Thus, when the metal pipe is set straight, the length of the optical fiber becomes larger than that of the metal pipe, so that the optical fiber flexes in the metal pipe, thereby preventing the optical fiber from causing a strain by the laying tension or the like.
Furthermore, when the metal pipe is damaged to form a hole, water can enter through the hole to degrade the optical fiber. In order to prevent this, a gel filler is injected into the metal pipe. More specifically, after the optical fiber is blown to the outer side within the metal pipe with the inert gas at the capstan, a filler is injected into the metal pipe through a filler introducing tube which is different from the introducing tube that introduces the optical fiber.
However, the conditions in which the fiber is used vary, and the optical fiber is used in various temperature conditions. The thermal expansion coefficient of the metal pipe as the sheath is greatly larger than that of the optical fiber. Hence, when the optical fiber is used in a high temperature, a tension is applied to the optical fiber due to a difference in elongation degree between the metal pipe and the optical fiber, thus causing damage to the optical fiber. A similar phenomenon also occurs when, e.g., a cable is laid underseas and thus placed with a large tension.
Inversely, when an optical cable is used at a low temperature, due to a difference in degree of shrinkage between the metal pipe and the optical fiber, the optical fiber is brought into contact with the inner wall of the metal pipe having a larger amount of shrinkage, and directly receives an edgewise pressure from the inner wall of the metal pipe, or irregular bents having small cycles are applied to the optical fiber, thus causing a so-called microbend loss. Then, the intensity of a signal transmitted through the optical fiber is attenuated.
In order to prevent these transmission loss and the like, conventionally, the optical fiber is blown to the outer side within the metal pipe while the metal pipe is engaged with the capstan, as described above, so that the length of the optical fiber becomes larger than that of the metal pipe when the metal pipe is set straight.
In this case, however, the difference in length between the optical fiber and the metal pipe (to be referred to as an extra length hereinafter) is determined by the outer diameter of the capstan and the difference between the inner diameter of the metal pipe and the outer diameter of the optical fiber. The extra length cannot thus be arbitrarily controlled, and a transmission loss can still occur in the optical fiber depending on the use conditions.
As described above, the optical fiber is blown to the outer side within the metal pipe with the inert gas while the metal pipe is engaged with the capstan, thereby imparting an extra length to the optical fiber. Therefore, when injecting a filler in the metal pipe, the filler must be injected while the optical fiber is blown to the outer side within the metal pipe. This is because of the following reason. Namely, if the filler is injected in advance and thereafter the inert gas is supplied, the filler serves as a resistance, and thus an extra length cannot be imparted to the optical fiber. Therefore, when injecting a filler, a filler introducing tube is required in addition to the introducing tube used for supplying the optical fiber and the inert gas. Since two introducing tubes must be separately inserted in the metal pipe, the inner diameter of the metal pipe must be large. Accordingly, the drawing amount required for drawing the metal pipe to reduce its diameter is increased. In some cases, the metal pipe cannot be drawn thin in accordance with the diameter of the optical fiber.
In view of the above situation, in order to solve the above problems, one of the present inventors proposes a method described in U.S. Pat. No. 5,231,260 and U.S. Ser. No. 08/078,394. According to the method disclosed in these official gazettes, the extra length can be arbitrarily controlled by adjusting the tension of the metal strip and the tension of the optical fiber. However, as an optical fiber is made of brittle glass, when excessive tension is applied to the optical fiber, a crack can be generated in the optical fiber. In other words, the service life of the optical fiber is shortened.